Low-voltage dry-type mini–power centers · Eaton’s mini–power centers are available with a tin-plated aluminum bus loadcenter interior that accepts 1-inch plug-on branch circuit

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Low-voltage dry-type mini–power centers

Contents

General Description . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 .3-2Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 .3-2General Description . . . . . . . . . . . . . . . . . . . . . . . . . . 19 .3-2Application Description . . . . . . . . . . . . . . . . . . . . . . . 19 .3-2Features, Benefits and Functions . . . . . . . . . . . . . . . . 19 .3-2

Technical Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 .3-3Primary Main Breakers . . . . . . . . . . . . . . . . . . . . . . . . 19 .3-3Distribution Loadcenter . . . . . . . . . . . . . . . . . . . . . . . 19 .3-3Dry-Type Encapsulated Transformers . . . . . . . . . . . . . 19 .3-3Typical Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 .3-4NEMA Type 3R Plug-In and Bolt-On Selection Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 .3-5NEMA Type 4X Plug-In and Bolt-On Selection Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 .3-7Dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 .3-9

Application Information . . . . . . . . . . . . . . . . . . . . . . . 19 .3-11Transformer Sizing . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 .3-11Standards and Certifications . . . . . . . . . . . . . . . . . . . 19 .3-13Glossary of Transformer Terms . . . . . . . . . . . . . . . . . . 19 .3-14Frequently Asked Questions About Transformers . . . 19 .3-17

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Design Guide DG009003EN Effective February 2020

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OverviewA mini–power center combines three individual components into one NEMA enclosure:

■ Primary main breaker■ Transformer■ Secondary distribution loadcenter with main breaker

All interconnecting wiring is performed at the factory.

General Description■ Encapsulated single-phase (Type EP) or three-phase (EPT) dry-type transformer

■ Suitable for indoor or outdoor applications

■ 180 °C insulation system■ 115 °C rise standard; ■ 80 °C rise optional■ Mini–power centers are UL listed in conformity with UL 1062

■ Mini–power centers are encapsulated transformers and are outside the scope of U.S. DOE energy efficiency requirements

Application DescriptionMini–power centers are used wherever there is a 480 V or 600 V distribution system and loads requiring 208Y/120 V, three-phase or 120/240 V single-phase. Typical installations include:

■ Industrial plant assembly lines■ Plant expansions■ Test equipment■ Temporary construction site power■ Sewage disposal plants■ Warehouses■ Car washes■ Parking lots■ Commercial buildings

Features, Benefits and Functions

■ 60 Hz operation■ Indoor/outdoor, NEMA 3R or NEMA 4X■ Primary main breaker■ Secondary main breaker (Type BR plug-in and Type BAB bolt-on branch breakers not included)

■ All live parts enclosed for personnel safety

■ Cover is hinged to prevent removal and can be padlocked

■ Cores are grounded with a copper lead■ Ground bar is supplied to permit grounding of individual secondary circuits

■ Neutral bar is grounded to case■ Branch circuits can be easily added or changed using Type BR plug-in and Type BAB bolt-on

■ Suitable for service entrance■ Wide range of configurations available:

❏ Aluminum windings and plug-in loadcenter chassis standard

❏ Copper windings and plug-in loadcenter chassis also available

❏ Copper windings and bolt-on loadcenter chassis standard

■ Short-term overload capability as required by ANSI

■ Meet NEMA ST-20 sound levels■ Eaton manufactured dry-type distribution transformers are seismically qualified and exceed requirements of the International Building Code (IBC) and California Code Title 24


19 .3-2

Low-Voltage Dry-TypeMini–Power Centers

EATON www.eaton.com

General Description

Primary Main BreakersMini–power centers are provided with primary main circuit breakers that have an interrupting rating of 14 kAIC. EHD primary main circuit breakers are provided as standard on mini–power centers with a 480 V primary; 600 V designs have an FD primary main breaker as standard. Primary main circuit breakers with greater AIC ratings are available as an option on mini–power centers with plug-on secondary circuit breakers.

Optional primary main breakers are FD (rated 25 kAIC at 480 V; 18 kAIC at 600 V), HFD (65 kAIC at 480 V; 25 kAIC at 600 V) or FDC (100 kAIC at 480 V; 35 kAIC at 600 V).

Distribution LoadcenterEaton’s mini–power centers are available with a tin-plated aluminum bus loadcenter interior that accepts 1-inch plug-on branch circuit breakers Type BR family.

The all-copper mini–power center is constructed with a copper-wound transformer and tin-plated copper bus loadcenter interior that accepts 1-inch bolt-on branch breakers Type BAB family.

Dry-Type Encapsulated Transformers

Core and Coil AssembliesThe transformer core is constructed using high-grade, non-aging, silicon steel with high-magnetic permeability, and low hysteresis and eddy current losses. Maximum magnetic flux densities are substantially below the saturation point. The transformer core volume allows for efficient transformer operation at 10% above the nominal tap voltage. The core laminations are tightly clamped and compressed. Coils are wound of electrical grade aluminum or copper, and are of continuous wound construction. The BIL (basic impulse level) for all 600 V-class windings is 10 kV. In encapsulated transformers, the core and coil assembly is completely encased in a proportioned mixture of resin or epoxy, and aggregate to provide a moisture-proof, shock-resistant seal. The core and coil encapsulation system is designed to minimize the audible sound level.

FrequencyEaton standard low-voltage dry-type distribution transformers are designed for 60 Hz operation. Transformers required for other frequencies are available and must be specifically designed.

Overload CapabilityShort-term overload is designed into transformers as required by ANSI. Low-voltage dry-type distribution transformers will deliver 200% nameplate load for one-half hour, 150% load for one hour and 125% load for four hours without being damaged, provided that a constant 50% load precedes and follows the overload. See ANSI C57.96-01.250 for additional limitations.

Continuous overload capacity is not deliberately designed into a transformer because the design objective is to be within the allowed winding temperature rise with nameplate loading.

Sound LevelsAll Eaton 600 volt class general-purpose dry-type distribution transformers are designed to meet NEMA ST-20 sound levels listed here. These are the sound levels measured in a soundproof environment. Actual sound levels measured at an installation will likely be higher due to electrical connections and environmental conditions. Lower sound levels are available and should be specified when the transformer is going to be installed in an area where sound may be a concern.

Table 19.3-1. Average Sound Levels NEMA ST-20 Average Sound Level, dB

Equivalent Winding kVA Range

Encapsulated (up to 1 .2 kV)

3.00 and below3.01 to 9.009.01 to 15.0015.01 to 30.00

45455050

Note: For other ratings not shown, or for special enclosure types (including stainless steel), refer to Eaton.

Insulation SystemsThe design life of transformers having different insulation systems is the same; the lower temperature systems are designed for the same life as the higher temperature systems.

Eaton’s encapsulated transformers are manufactured using a 180 °C insulation system. Required performance is obtained without exceeding the insulation system rating at rated temperature rise in a 40 °C maximum ambient, with an average ambient temperature of 30 °C over a 24-hour period.

All insulation materials are flame retardant and do not support combustion as defined in ASTM Standard Test Method D635.


19 .3-3


EATON www.eaton.com

Technical Data

Typical Diagrams

Figure 19.3-1. Single-Phase Mini–Power Center

Figure 19.3-2. Three-Phase Mini–Power Center

LV

HV

2

4

X1

H4

X3

H3

2

2625

1

3 2324

22

3

X4X2

H2

1

H1

26 25

22

24

21

23

4

21

X0 X1 X2 X3

A B A B A B

PHASE A PHASE B PHASE C

H1 H2

2 3 41 2 3 41

119753 13 15 17 19 21 23H3 11

2

2 3 41 X1

X2

X3

1064 8 12 14 16

2132 20 2422 23

2418 20 22


19 .3-4

Low-Voltage Dry-TypeMini–Power CentersTechnical Data

EATON www.eaton.com

NEMA Type 3R Plug-In and Bolt-On Selection Tables

Table 19.3-2. Plug-In Mini–Power Center (Aluminum-Wound Transformer and Loadcenter Chassis Standard) Catalog Number Information—NEMA Type 3RkVA Catalog

NumberFull Capacity Taps FCBN

Dimensions in Inches (mm) a Weight Frame Main Circuit Breaker b Branch Breakers Max . Number de

Max . Amp

Height Width Depth Lb (kg) Primary c Secondary Single- Pole

Two- Pole

Three- Pole

Single-Phase

480 V to 120/240 V

3 5 7.5

P48G11S0312P48G11S0512P48G11S0712

2 at –5%2 at –5%2 at –5%

29.50 (749.3)29.50 (749.3)29.50 (749.3)

11.90 (302.3)11.90 (302.3)11.90 (302.3)

8.90 (226.1) 8.90 (226.1) 8.90 (226.1)

125 (56)125 (56)125 (56)

FR284FR284FR284

EHD2015EHD2020EHD2030

BR215BR225BR230

121212

6 6 6

———

12 20 30

101525

P48G11S1020P48G11S1526P48G11S2526

2 at –5%2 at –5%2 at –5%

37.50 (952.5)43.40 (1102.4)43.40 (1102.4)

13.10 (332.7)15.90 (403.9)15.90 (403.9)

11.60 (294.6)14.50 (368.3)14.50 (368.3)

212 (96)373 (169)373 (169)

FR286FR287FR287


BR250BR270BR2125

202626

101313

———

40 60100

600 V to 120/240 V 5 7.510

P60G11S0512P60G11S0712P60G11S1020

2 at –5%2 at –5%2 at –5%

29.50 (749.3)29.50 (749.3)37.50 (952.5)

11.90 (302.3)11.90 (302.3)13.10 (332.7)

8.90 (226.1) 8.90 (226.1)11.60 (294.6)

125 (56)125 (56)212 (96)

FR284FR284FR286

FDB2015FDB2030FDB2040

BR225BR230BR250

121220

6 610

———

20 30 40

1525

P60G11S1526P60G11S2526

2 at –5%2 at –5%

43.40 (1102.4)43.40 (1102.4)

15.90 (403.9)15.90 (403.9)

14.50 (368.3)14.50 (368.3)

373 (169)373 (169)

FR287FR287

FDB2060FDB2100

BR270BR2125

2626

1313

——

60100

Three-Phase

480 V to 208Y/120 V1522.530

P48G28T1518P48G28T2124P48G28T3024

2 at –5%2 at –5%2 at –5%

36.10 (916.9)40.01 (1016.3)40.01 (1016.3)

28.80 (731.5)32.63 (828.8)32.63 (828.8)

9.40 (238.8)13.62 (345.9)13.62 (345.9)

320 (145)635 (288)635 (288)

FR289BFR291BFR291B



182424

91212

688

40 60 80

600 V to 208Y/120 V1522.530

P60G28T1518P60G28T2124P60G28T3024

2 at –5%2 at –5%2 at –5%

36.10 (916.9)40.01 (1016.3)40.01 (1016.3)

28.80 (731.5)32.63 (828.8)32.63 (828.8)

9.40 (238.8)13.62 (345.9)13.62 (345.9)

320 (145)635 (288)635 (288)

FR289BFR291BFR291B



182424

91212

688

40 60 80

a Not for construction purposes.b Secondary main breakers fixed only. No substitutes for ampere rating.c Optional AIC rated main breakers available. .d Combinations can be selected.e Branch breakers not included. Use Eaton’s Type BR family.


19 .3-5


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Table 19.3-3. Bolt-On Mini–Power Center (Copper-Wound Transformer and Loadcenter Chassis Standard) Catalog Number Information—NEMA Type 3RkVA Catalog


Dimensions in inches (mm) a Weight Frame Main Circuit Breaker b Branch Breakers Max . Number de

Max . Amp


Two- Pole

Three-Pole

Single-Phase

480 V to 120/240 V

3 5 7.5

P48G11S0318CUBP48G11S0518CUBP48G11S0718CUB

2 at –5%2 at –5%2 at –5%

36.14 (918.0)36.14 (918.0)36.14 (918.0)

12.56 (319.0)12.56 (319.0)12.56 (319.0)

9.66 (245.4) 9.66 (245.4) 9.66 (245.4)

110 (50)110 (50)110 (50)

FR307FR307FR307


BAB2015BAB2025BAB2030

181818

9 9 9

———

12 20 30

101525


2 at –5%2 at –5%2 at –5%

43.91 (1115.3)43.37 (1101.6)43.37 (1101.6)

14.97 (380.2)20.41 (518.4)20.41 (518.4)

11.82 (300.2)14.58 (370.3)14.58 (370.3)

215 (98)385 (175)385 (175)

FR309FR310FR310



243030

121515

———

40 60100

600 V to 120/240 V 3 5 7.5


2 at –5%2 at –5%2 at –5%

36.14 (918.0)36.14 (918.0)36.14 (918.0)

12.56 (319.0)12.56 (319.0)12.56 (319.0)

9.66 (245.4) 9.66 (245.4) 9.66 (245.4)

110 (50)110 (50)110 (50)

FR307FR307FR307



181818

9 9 9

———

12 20 30

101525


2 at –5%2 at –5%2 at –5%

43.91 (1115.3)43.37 (1101.6)43.37 (1101.6)

14.97 (380.2)20.41 (518.4)20.41 (518.4)

11.82 (300.2)14.58 (370.3)14.58 (370.3)

215 (98)385 (175)385 (175)

FR309FR310FR310



243030

121515

———

40 60100

Three-Phase

480 V to 208Y/120 V1522.530

P48G28T1518CUBP48G28T2124CUBP48G28T3024CUB

2 at –5%2 at –5%2 at –5%

34.27 (870.5)40.01 (1016.3)40.01 (1016.3)

31.50 (800.1)32.63 (828.8)32.63 (828.8)

9.35 (237.5)13.62 (345.9)13.62 (345.9)

320 (145)635 (288)635 (288)

FR289BFR291BFR291B


BAB3050HBAB3070HBAB3100H

182424

91212

688

40 60 80

600 V to 208Y/120 V1522.530

P60G28T1518CUBP60G28T2124CUBP60G28T3024CUB

2 at –5%2 at –5%2 at –5%

34.27 (870.5)40.01 (1016.3)40.01 (1016.3)

31.50 (800.1)32.63 (828.8)32.63 (828.8)

9.35 (237.5)13.62 (345.9)13.62 (345.9)

320 (145)635 (288)635 (288)

FR289BFR291BFR291B



182424

91212

688

40 60 80

a Not for construction purposes.b Main breakers fixed only. No substitutes for ampere rating.c Optional AIC rated main breakers available. d Combinations can be selected.e Branch breakers not included. Use Eaton’s Type BAB family.


19 .3-6


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NEMA Type 4X Plug-In and Bolt-On Selection Tables

Table 19.3-4. Plug-In Mini–Power Center (Aluminum-Wound Transformer and Loadcenter Chassis Standard) Catalog Number Information—NEMA Type 4XkVA Catalog



Max . Amp


Two- Pole

Three- Pole

Single-Phase

480 V to 120/240 V

3 5 7.5

P48G11S0312S64XP48G11S0512S64XP48G11S0712S64X

2 at –5%2 at –5%2 at –5%

34.25 (870.0)34.25 (870.0)34.25 (870.0)

16.00 (406.4)16.00 (406.4)16.00 (406.4)

11.35 (288.3)11.35 (288.3)11.35 (288.3)

306 (139)306 (139)306 (139)

FR2854XFR2854XFR2854X


BR215BR225BR230

121212

6 6 6

———

12 20 30

101525


2 at –5%2 at –5%2 at –5%

39.81 (1011.2)39.81 (1011.2)39.81 (1011.2)

19.74 (501.4)19.74 (501.4)19.74 (501.4)

15.94 (404.9)15.94 (404.9)15.94 (404.9)

546 (248)546 (248)546 (248)



BR250BR270BR2125

202626

101313

———

40 60100

600 V to 120/240 V 5 7.510


2 at –5%2 at –5%2 at –5%

34.25 (870.0)34.25 (870.0)39.81 (1011.2)

16.00 (406.4)16.00 (406.4)19.74 (501.4)

11.35 (288.3)11.35 (288.3)15.94 (404.9)

306 (139)306 (139)546 (248)



BR225BR230BR250

121220

6610

———

20 30 40

1525

P60G11S1526S64XP60G11S2526S64X

2 at –5%2 at –5%

39.81 (1011.2)39.81 (1011.2)

19.74 (501.4)19.74 (501.4)

15.94 (404.9)15.94 (404.9)

546 (248)546 (248)

FR2874XFR2874X

FDB2060FDB2100

BR270BR2125

2626

1313

——

60100

Three-Phase

480 V to 208Y/120 V1522.530

P48G28T1524S64XP48G28T2124S64XP48G28T3024S64X

2 at –5%2 at –5%2 at –5%

37.63 (955.8)37.63 (955.8)37.63 (955.8)

30.84 (783.3)30.84 (783.3)30.84 (783.3)

14.50 (368.3)14.50 (368.3)14.50 (368.3)

728 (330)728 (330)728 (330)




242424

121212

888

406080

600 V to 208Y/120 V1522.530

P60G28T1524S64XP60G28T2124S64XP60G28T3024S64X

2 at –5%2 at –5%2 at –5%

37.63 (955.8)37.63 (955.8)37.63 (955.8)

30.84 (783.3)30.84 (783.3)30.84 (783.3)

14.50 (368.3)14.50 (368.3)14.50 (368.3)

728 (330)728 (330)728 (330)




242424

121212

888

406080

a Not for construction purposes.b Secondary main breakers fixed only. No substitutes for ampere rating.c Optional AIC rated main breakers available. d Combinations can be selected.e Branch breakers not included. Use Eaton’s Type BR family.


19 .3-7


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Table 19.3-5. Bolt-On Mini–Power Center (Copper-Wound Transformer and Loadcenter Chassis Standard) Catalog Number Information—NEMA Type 4XkVA Catalog



Max . Amp


Two- Pole

Three-Pole

Single-Phase

480 V to 120/240 V

3 5 7.5

P48G11S0318CUBS64XP48G11S0518CUBS64XP48G11S0718CUBS64X

2 at –5%2 at –5%2 at –5%

34.25 (870.0)34.25 (870.0)34.25 (870.0)

16.00 (406.4)16.00 (406.4)16.00 (406.4)

11.35 (288.3)11.35 (288.3)11.35 (288.3)

340 (154)340 (154)340 (154)




181818

9 9 9

———

12 20 30

101525


2 at –5%2 at –5%2 at –5%

34.25 (870.0)37.63 (955.8)37.63 (955.8)

16.00 (406.4)30.84 (783.3)30.84 (783.3)

11.35 (288.3)14.50 (368.3)14.50 (368.3)

340 (154)650 (295)650 (295)




243030

121515

———

40 60100

600 V to 120/240 V 3 5 7.5


2 at –5%2 at –5%2 at –5%

34.25 (870.0)34.25 (870.0)34.25 (870.0)

16.00 (406.4)16.00 (406.4)16.00 (406.4)

11.35 (288.3)11.35 (288.3)11.35 (288.3)

340 (154)340 (154)340 (154)




181818

9 9 9

———

12 20 30

101525


2 at –5%2 at –5%2 at –5%

34.25 (870.0)37.63 (955.8)37.63 (955.8)

16.00 (406.4)30.84 (783.3)30.84 (783.3)

11.35 (288.3)14.50 (368.3)14.50 (368.3)

340 (154)650 (295)650 (295)




243030

121515

———

60 60100

Three-Phase

480 V to 208Y/120 V1522.530

P48G28T1524CUBS64XP48G28T2124CUBS64XP48G28T3024CUBS64X

2 at –5%2 at –5%2 at –5%

37.63 (955.8)37.63 (955.8)37.63 (955.8)

30.84 (783.3)30.84 (783.3)30.84 (783.3)

14.50 (368.3)14.50 (368.3)14.50 (368.3)

898 (407)898 (407)898 (407)




242424

121212

888

40 60 80

600 V to 208Y/120 V1522.530

P60G28T1524CUBS64XP60G28T2124CUBS64XP60G28T3024CUBS64X

2 at –5%2 at –5%2 at –5%

37.63 (955.8)37.63 (955.8)37.63 (955.8)

30.84 (783.3)30.84 (783.3)30.84 (783.3)

14.50 (368.3)14.50 (368.3)14.50 (368.3)

898 (407)898 (407)898 (407)




242424

121212

888

40 60 80

a Not for construction purposes.b Main breakers fixed only. No substitutes for ampere rating.c Optional AIC rated main breakers available. d Combinations can be selected.e Branch breakers not included. Use Eaton’s Type BAB family.


19 .3-8


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Dimensions

Figure 19.3-3. Enclosure Dimensional Drawings—Mini–Power Centers—NEMA Type 3R

Table 19.3-6. Mini–Power Centers—Approximate Dimensions in Inches (mm)Frame Drawing

NumberDimensions

Height Width Depth

FR284FR286FR287FR289B

1112

29.50 (749)38.13 (968)43.88 (1115)34.27 (871)

12.56 (319)13.47 (342)16.40 (417)31.50 (800)

9.78 (248)11.87 (301)14.63 (372) 9.35 (237)

FR291BFR307FR309FR310

2111

40.01 (1016)36.14 (918)43.91 (1115)43.37 (1102)

32.63 (829)12.56 (319)14.97 (380)20.41 (518)

13.62 (346) 9.66 (245)11.82 (300)14.58 (370)


19 .3-9


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Figure 19.3-4. Enclosure Dimensional Drawings —Mini–Power Centers—NEMA Type 4X

Table 19.3-7. Mini–Power Centers—Approximate Dimensions in Inches (mmFrame Drawing

NumberDimensions

Height Width Depth


333

36.32 (922)42.07 (1069)39.88 (1013)

22.01 (559)25.75 (654)36.85 (936)

14.35 (365)18.94 (481)17.50 (445)


19 .3-10


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Transformer Sizing

Single-Phase Transformers

How to Select Single-Phase Units1. Determine the primary (source)

voltage—the voltage presently available.

2. Determine the secondary (load) voltage—the voltage needed at the load.

3. Determine the kVA load: —If the load is defined in kVA, a transformer can be selected from the tabulated data.

—If the load rating is given in amperes, determine the load kVA from the following chart. To deter mine kVA when volts and amperes are known, use the formula:

—If the load is an AC motor, determine the minimum trans former kVA from Table 19 .3-8 at the right.

—Select a transformer rating equal to or greater than the load kVA.

4. Define tap arrangements needed.

5. Define temperature rise.

Table 19.3-8. Single-Phase AC MotorsNote: When motor service factor is greater than 1, increase full load amperes proportionally. Example: If service factor is 1.15, increase ampere values by 15%.

Horsepower Full Load Amperes MinimumTransformer kVA a115 Volts 208 Volts 220 Volts 230 Volts

1/6 1/4 1/3

4.4 5.8 7.2

2.4 3.2 4.0

2.3 3.0 3.8

2.2 2.9 3.6

0.53 0.70 0.87

1/2 3/4 1

9.8 13.8 16

5.4 7.6 8.8

5.1 7.2 8.4

4.9 6.9 8

1.18 1.66 1.92

1-1/2 2 3

20 24 34

11.013.218.7

10.412.517.8

101217

2.40 2.88 4.10

5 7-1/210

56 80100

30.84455

29.34252

284050

6.72 9.612.0

a If motors are started more than once per hour, increase minimum transformer kVA by 20%.

Table 19.3-9. Full Load Current in Amperes—Single-Phase Circuits b kVA Voltage

120 208 220 240 277 480 600

3 5 7.5

25 41 62

14.4 24.0 36

13.6 22.7 34

12.5 20.8 31

10.818.027

6.210.415.6

5.0 8.312.5

101525

83125208

48 72120

45 68114

41 62104

365490

20.83152

16.72541

b Table of standard transformer ratings used to power single-phase motors in Table 19 .3-8.

kVA = Volts x Amperes 1000


19 .3-11


EATON www.eaton.com

Application Information

Three-Phase Transformers

How to Select Three-Phase Units1. Determine the primary (source)

voltage—the voltage presently available.

2. Determine the secondary (load) voltage—the voltage needed at the load.

3. Determine the kVA load: — If the load is defined in kVA, a transformer can be selected from the tabulated data.

— If the load rating is given in amperes, determine the load kVA from the following chart. To deter mine kVA when volts and amperes are known, use the formula:

— If the load is an AC motor, deter mine the minimum transformer kVA from Table 19 .3-10 at the right.

— Select a transformer rating equal to or greater than the load kVA.

4. Define tap arrangements needed.

5. Define temperature rise.

Using the above procedure, select the transformer from the listings in this catalog.

Table 19.3-10. Three-Phase AC MotorsHorsepower Full Load Amperes Minimum

Transformer kVA a208 Volts 230 Volts 380 Volts 460 Volts 575 Volts

1/2 3/4 1

2.2 3.1 4.0

2.0 2.8 3.6

1.2 1.7 2.2

1.0 1.4 1.8

0.8 1.1 1.4

0.9 1.2 1.5

1-1/2 2 3

5.7 7.5 10.7

5.2 6.8 9.6

3.1 4.1 5.8

2.6 3.4 4.8

2.1 2.7 3.9

2.1 2.7 3.8

5 7-1/2 10

16.7 24 31

15.2 22 28

9.2 14 17

7.6 11 14

6.1 9 11

6.3 9.2 11.2

15 20 25

46 59 75

42 54 68

26 33 41

21 27 34

17 22 27

16.6 21.6 26.6

30 40 50

88114143

80104130

48 63 79

40 52 65

32 41 52

32.4 43.2 52

a If motors are started more than once per hour, increase minimum transformer kVA by 20%.Note: When motor service factor is greater than 1, increase full load amperes proportionally. Example: If service factor is 1.15, increase above ampere values by 15%.

Table 19.3-11. Full Load Current in Amperes—Three-Phase CircuitskVA Voltage

208 240 380 480 600

15 22.5 30

41.7 62.4 83.4

36.1 54.1 72.3

22.8 34.2 45.6

18.0 27.1 36.1

14.4 21.6 28.9

kVA = Volts x Amperes x 1 .732 1000


19 .3-12

Low-Voltage Dry-TypeMini–Power CentersApplication Information

EATON www.eaton.com

Standards and CertificationsEaton dry-type distribution transformers are approved, listed, recognized or may comply with the following standards.

Table 19.3-12. Engineering StandardsCatalog Product Name

UL Standard a

UL/cUL File Number

UL Listed Control Number

cUL Energy Efficiency Verification File Number

CSA File Number

Insulation System Temp/ºC

kVASingle- Phase

kVAThree- Phase

Applicable IEC Standard

Industrial Control TransformerMTE 5085 E46323 702X — — 105 0.025–1.5 N/A 61558

MTE 5085 E46323 702X — — 180 0.05–5 N/A 61558

Encapsulated TransformerAP 5085 E10156 591H — — 180 3–10 N/A 61558

AP 1561 E78389 591H — — 180 15 N/A 61558

EP 5085 E10156 591H — LR60545 180 0.05–10 N/A 61558

EP 1561 E78389 591H — LR60545 b 180 15–37.5 N/A 61558 c / 726 d

EPT 5085 E10156 591H — LR60545 180 N/A 3–9 61558 e / 726 f

EP 1561 E78389 591H — LR60545 g 180 N/A 15–75 726

MPC 1062 E53449 591H — LR60546 180 3–25 15–30 —

Ventilated TransformerDS-3 1561 E78389 591H EV33871 h — 220 7.5–167 N/A 60726

DS-3 1561 E78389 591H EV33871 i — 220 N/A 7.5–750 60726

KT 1561 E78389 591H EV33871 i — 220 N/A 7.5–750 N/A

a UL 5085 replaces UL 506.b Applies to 25 kVA.c Applies to 15–25 kVA.d Applies to 37.5 kVA.e Applies to 3 kVA.f Applies to 5–9 kVA.g Applies to 30 kVA.h Applies to 15–167 kVA.i Applies to 15–300 kVA.

In addition to the above standards, Eaton dry-type distribution transformers are also manufactured in compliance with the applicable standards listed below.

Not all of the following standards apply to every transformer.

NEC: National Electrical Code.

NEMA ST-1: Specialty Transformers (C89.1) (control transformers).

NEMA ST-20: General-Purpose Transformers.

DOE 2016 Final Rule: CFR Title 10 Chapter II Part 431, Appendix A of Subpart K 2016.

NEMA 250: Enclosures for Electrical Equipment (1000 volts maximum).

IEEE C57 .12 .01: General Requirements for Dry-Type Distribution and Power Transformers (including those with solidcast and/or resin-encapsulated windings).

ANSI C57 .12 .70: Terminal Markings and Connections for Distribution and Power Transformers.

ANSI C57 .12 .91: Standard Test Code for Dry-Type Distribution and Power Transformers.

CSA C22 No . 47-M90: Air-Cooled Transformers (Dry-Type).

CSA C9-M1981: Dry-Type Transformers.

CSA C22 .2 No . 66: Specialty Transformers.

CSA 802-94: Maximum Losses for Distribution, Power and Dry-Type Transformers.

NEMA TP-1: Guide for Determining Energy Efficiency for Distribution Transformers (rescinded).

NEMA TP-2: Standard Test Method for Measuring the Energy Consumption of Distribution Transformers (rescinded).

NEMA TP-3: Standard for the Labeling of Distribution Transformer Efficiency (rescinded).


19 .3-13


EATON www.eaton.com

Glossary of Transformer TermsAir cooled: A transformer that is cooled by the natural circulation of air around, or through, the core and coils.

Ambient noise level: The existing or inherent sound level of the area surrounding the transformer, prior to energizing the transformer. Measured in decibels.

Ambient temperature: The temperature of the air surrounding the transformer into which the heat of the transformer is dissipated.

Ampacity: The current-carrying capacity of an electrical conductor under stated thermal conditions. Expressed in amperes.

Ampere: The practical unit of electric current.

Attenuation: A decrease in signal power or voltage. Unit of measure is dB.

Autotransformer: A transformer in which part of the winding is common to both the primary and the secondary circuits.

Banked: Two or more single-phase transformers wired together to supply a three-phase load. Three single-phase transformers can be “banked” together to support a three-phase load. For example, three 10 kVA single-phase transformers “banked” together will have a 30 kVA three-phase capacity.

BIL: Basic impulse level. The ability of a transformer’s insulation system to withstand high voltage surges. All Eaton 600 V-class transformers have a 10 kV BIL rating.

BTU: British thermal unit. In North America, the term “BTU” is used to describe the heat value (energy content) of fuels, and also to describe the power of heating and cooling systems, such as furnaces, stoves, barbecue grills and air conditioners. When used as a unit of power, BTU “per hour” (BTU/h) is understood, though this is often abbreviated to just “BTU.”

Buck-Boost: The name of a standard, single-phase, two-winding transformer application with the low-voltage secondary windings connected as an autotransformer for boosting (increasing) or bucking (decreasing) voltages in small amounts. Applications can either be single-phase or three-phase.

CE: Mark to indicate third-party approved or self-certification to specific requirements of the European community.

Celsius (centigrade): Metric temperature measure.

°F = (1.8 x °C) + 32

°C = (°F-32) / 1.8

Center tap: A reduced capacity tap at the mid-point of a winding. The center tap on three-phase delta-delta transformers is called a lighting tap. It provides 5% of the transformer’s kVA for single-phase loads.

Certified tests: Actual values taken during production tests and certified as applying to a given unit shipped on a specific order. Certified tests are serial number–specific.

Common mode: Electrical noise or voltage fluctuation that occurs between all of the line leads and the common ground, or between ground and line or neutral.

Compensated transformer: A transformer with a turns ratio that provides a higher than nameplate output (secondary) voltage at no load, and nameplate output (secondary) voltage at rated load. It is common for small transformers (2 kVA and less) to be compensated.

Conductor losses: Losses (expressed in watts) in a transformer that are incidental to carrying a load: coil resistance, stray loss due to stray fluxes in the windings, core clamps, and the like, as well as circulating currents (if any) in parallel windings. Also called load losses.

Continuous duty rating: The load that a transformer can handle indefinitely without exceeding its specified temperature rise.

Core losses: Losses (expressed in watts) caused by magnetization of the core and its resistance to magnetic flux. Also called no-load losses or excitation losses. Core losses are always present when the transformer is energized.

CSA: Canadian Standards Association. The Canadian equivalent of Underwriters Laboratories (UL).

CSL3: Candidate Standard Level 3 (CSL3) design criteria developed by the U.S. Department of Energy.

cUL: Mark to indicate UL Certification to specific CSA Standards.

Decibel (dB): Unit of measure used to express the magnitude of a change in signal or sound level.

Delta connection: A standard three-phase connection with the ends of each phase winding connected in series to form a closed loop with each phase 120 degrees from the other. Sometimes referred to as three-wire.

Dielectric tests: Tests that consist of the application of a voltage higher than the rated voltage for a specified time for the purpose of determining the adequacy against breakdowns of insulating materials and spacings under normal conditions.

DOE 2016 efficient: A revision to federal law 10 CFR Part 431 (2007) that mandates higher efficiency for distribution transformers manufactured for sale in the U.S. and U.S. Territories effective January 1, 2016. “TP-1” efficient transformers can no longer legally be manufactured for use in the U.S. as of this date.

Dry-type transformer: A transformer in which the core and coils are in a gas eous or dry compound insulating medium. A transformer that is cooled by a medium other than a liquid, normally by the circulation of air.

Eddy currents: The currents that are induced in the body of a conducting mass by the time variation of magnetic flux or varying magnetic field.

Efficiency: The ratio of the power output from a transformer to the total power input. Typically expressed as a %.

Electrostatic shield: Copper or other conducting sheet placed between primary and secondary windings, and grounded to reduce electrical interference and to provide additional protection from line-to-line or line-to-ground noise. Commonly referred to as “Faraday shield.”

Encapsulated transformer: A transformer with its coils either dipped or cast in an epoxy resin or other encapsulating substance.

Enclosure: A surrounding case or housing used to protect the contained equipment against external conditions and prevent personnel from accidentally contacting live parts.

Environmentally preferable product: A product that has a lesser or reduced negative effect on human health and the environment when compared to competing products that serve the same purpose. This comparison may consider raw materials acquisition, production, manufacturing, packaging, distribution, reuse, operation, maintenance and disposal of the product. This term includes recyclable products, recycled products and reusable products.


19 .3-14


EATON www.eaton.com

EPACT: The Energy Policy Act of 1992 (EPAct) is an important piece of legislation for efficiency because it established minimum efficiency levels for dry-type distribution transformers manufactured or imported after December 2006. EPAct, which was based on NEMA standards, defined a number of terms, including what constitutes an energy-efficient transformer. The DOE issued a rule that defines these transformers and how manufacturers must comply. In April 2013, the DOE mandated even higher minimum efficiency levels for distribution transformers effective starting in January 2016. DOE EPAct rule (PDF): Energy Efficiency Program for Certain Commercial and Industrial Equipment: Test Procedures, Labeling, and the Certification Requirements for Electric Motors. Final Rule. 10-CFR Part 431.

Excitation current: No load current. The current that flows in any winding used to excite the transformer when all other windings are open-circuited. It is usually expressed in percent of the rated current of a winding in which it is measured. Also called magnetizing current.

FCAN: “Full Capacity Above Nominal” taps. Designates the transformer will deliver its rated kVA when connected to a voltage source which is higher than the rated primary voltage.

FCBN: “Full Capacity Below Nominal” taps. Designates the transformer will deliver its rated kVA when connected to a voltage source which is lower than the rated primary voltage.

Frequency: On AC circuits, designates the number of times that polarity alternates from positive to negative and back again per second, such as 60 cycles per second. Typically measured in Hertz (Hz).

Ground: Connecting one side of a circuit to the earth through low resistance or low impedance paths to help prevent transmitting electrical shock to personnel.

Harmonic: A sinusoidal waveform with a frequency that is an integral multiple of the fundamental frequency (60 Hz).

60 H3 fundamental 120 H3 2nd harmonic 180 H3 3rd harmonic 240 H3 4th harmonic

Harmonic distortion: Nonlinear distortion of a system characterized by the appearance of harmonic (non-sinusoidal) currents in the output, when the input is sinusoidal.

Harmonic distortion, total (THD): The square root of the sum of the squares of all harmonic currents present in a load, excluding the fundamental 60 Hz current. Usually expressed as a percent of the fundamental.

High-voltage windings: In a two-winding transformer, the winding intended to have the greater voltage. Usually marked with “H” designations.

HMT: Harmonic Mitigating Transformer (HMT) is better able to handle the harmonic currents present in today’s electrical power system. thereby increasing system capacity, reducing distortion throughout a facility, help to minimize downtime and “mysterious” maintenance on equipment, and return the longevity of equipment life through reduced operational energy losses, thereby running cooler.

Hp: Horsepower. The energy required to raise 33,000 pounds a distance of one foot in one minute. 1 hp is equal to 746 watts, or 0.746 kW.

Hi pot: A standard test on dry-type transformers consisting of extra-high potentials (voltages) connected to the windings. Used to check the integrity of insulation materials and clearances.

Hottest-spot temperature: The high est temperature inside the transformer winding. Is greater than the measured average temperature of the coil conductors, when using the resistance change method.

Hysteresis: The tendency of a magnetic substance to persist in any state of magnetization.

Impedance: The retarding forces of current in an AC circuit; the current-limiting characteristics of a transformer. Symbol = Z

Inductance: In electrical circuits, the opposition to a change in the flow of electrical current. Symbol = L

Inducted potential test: A standard dielectric test of transformer insulation. Verifies the integrity of insulating materials and electrical clearances.

Inrush current: The initial high peak of current that occurs in the first few cycles of energization, which can be 30 to 40 times the rated current.

Insulating transformer: Another term for an isolating transformer.

Insulation: Material with a high electrical resistance.

Insulation materials: Those materials used to insulate the transformer’s electrical windings from each other and ground.

Integral TVSS or SPD: Major standard change for surge protective devices (formerly known as transient voltage surge suppressors). The primary safety standard for transient voltage surge suppressors (TVSS) has under gone major revisions in the past three years with mandatory compli ance by manufacturers required by September 29, 2009. Even the name of the standard has changed from UL Standard for Safety for Transient Voltage Surge Suppressors, UL 1449 to UL Standard for Safety for Surge Protective Devices, UL 1449. This means that TVSS listed to the UL 1449 2nd Edition standard will no longer be able to be manufactured after September 29, 2009. All Surge Protective Devices must be designed, tested, manufactured and listed to the UL 1449 3rd Edition standard after this date.

Isolating transformer: A transformer where the input (primary) windings are not connected to the output (secondary) windings (i.e., electrically isolated).

K-factor: A common industry term for the amount of harmonics produced by a given load. The larger the K-factor, the more harmonics that are present. Also used to define a transformer’s abil ity to withstand the additional heating generated by harmonic currents.

kVA: Kilovolt-ampere. Designates the output that a transformer can deliver for a specified time at a rated secondary voltage and rated frequency without exceeding the specified temperature rise. When multiplied by the power factor, will give kilowatts or kW.

1000 VA = 1 kVA

Lamination: Thin sheets of electrical steel used to construct the core of a transformer.

Limiting temperature: The maximum temperature at which a component or material may be operated continuously with no sacrifice in normal life expectancy.

Linear load: A load where the current waveform conforms to that of the applied voltage, or a load where a change in current is directly proportional to a change in applied voltage.

Live part: Any component consisting of an electrically conductive material that can be energized under conditions of normal use.


19 .3-15


EATON www.eaton.com

Load losses: I2R losses in windings. Also see conductor losses.

Low-voltage winding: In a two-winding transformer, the winding intended to have the lesser voltage. Usually marked with “X” designations.

Mid-tap: See center tap.

Noise level: The relative intensity of sound, measured in decibels (dB). NEMA Standard ST-20 outlines the maximum allowable noise level for dry-type transformers.

Nonlinear load: A load where the current waveform does not conform to that of the applied voltage, or where a change in current is not proportional to a change in applied voltage.

Non-ventilated transformer: A transformer where the core and coil assembly is mounted inside an enclosure with no openings for ventilation. Also referred to as totally enclosed non-ventilated (TENV).

No load losses: Losses in a transformer that is excited at rated voltage and frequency but that is not supplying a load. No load losses include core losses, dielectric losses and conductor losses in the winding due to the exciting current. Also referred to as excitation losses.

Overload capability: Short-term overload capacity is designed into transformers as required by ANSI. Continuous overload capacity is not deliberately designed into a transformer because the design objective is to be within the allowed winding temperature rise with nameplate loading.

Percent IR (% resistance): Voltage drop due to resistance at rated current in percent of rated voltage.

Percent IX (% reactance): Voltage drop due to reactance at rated current in percent of rated voltage.

Percent IZ (% impedance): Voltage drop due to impedance at rated current in percent of rated voltage.

Phase: Type of AC electrical circuit; usually single-phase two- or three-wire, or three-phase three- or four-wire.

Polarity test: A standard test on transformers to determine instantaneous direction of the voltages in the primary compared to the secondary.

Primary taps: Taps added to the primary (input) winding. See Tap.

Primary voltage: The input circuit voltage.

Power factor: The cosine of the phase angle between a voltage and a current.

Ratio test: A standard test of transformers to determine the ratio of the input (primary) voltage to the output (secondary) voltage.

Reactance: The effect of inductive and capacitive components of a circuit producing other than unity power factor.

Reactor: A single winding device with an air or iron core that produces a specific amount of inductive reactance into a circuit. Normally used to reduce of control current.

Regulation: Usually expressed as the percent change in output voltage when the load goes from full load to no load.

Scott T connection: Connection for three-phase transformers. Instead of using three sets of coils for a three-phase load, the transformer uses only two sets of coils.

Series/multiple winding: A winding consisting of two or more sections that can be connected for series operation or multiple (parallel) operation. Also called series-parallel winding.

Short circuit: A low resistance connection, usually accidental, across part of a circuit, resulting in excessive current flow.

Sound levels: All transformers make some sound mainly due to the vibration generated in its core by alternating flux. All Eaton general-purpose dry-type distribution transformers are designed with sound levels lower than NEMA ST-20 maximum levels.

Star connection: Same as a wye connection.

Step-down transformer: A trans former where the input voltage is greater than the output voltage.

Step-up transformer: A transformer where the input voltage is less than the output voltage.

T-T connection: See Scott T connection.

Tap: A connection brought out of a winding at some point between its extremities, usually to permit changing the voltage or current ratio. Taps are typically used to compensate for above or below rated input voltage, in order to provide the rated output voltage. See FCAN and FCBN.

Temperature class: The maximum tem perature that the insulation system of a transformer can continuously with stand. The common insulation classes are 105, 150, 180 (also 185) and 220.

Temperature rise: The increase over ambient temperature of the windings due to energizing and loading the transformer.

Total losses: The sum of the no-load losses and load losses.

Totally enclosed non-ventilated enclosure: The core and coil assembly is installed inside an enclosure that has no ventilation to cool the transformer. The transformer relies on heat to radiate from the enclosure for cooling.

Transformer tests: Per NEMA ST-20, routine transformer production tests are performed on each transformer prior to shipment. These tests are: Ratio tests on the rated voltage connection; Polarity and Phase Relation tests on the rated connection; No-Load and Excitation Current tests at rated voltage on the rated voltage connection and Applied Potential and Induced Potential tests. Special tests include sound level testing.

Transverse mode: Electrical noise or voltage disturbance that occurs between phase and neutral, or from spurious signals across metallic hot line and the neutral conductor.

Turns ratio: The ratio of the number of turns in the high voltage winding to that in the low voltage winding.

Typical test data: Tests that were performed on similar units that were previously manufactured and tested.

UL (Underwriters Laboratories): An independent safety testing organization.

Universal taps: A combination of six primary voltage taps consisting of 2 at +2-1/2% FCAN and 4 at -2-1/2% FCBN.

Watt: A unit of electrical power when the current in a circuit is one ampere and the voltage is one volt.

Wye connection: A standard three-wire transformer connection with similar ends of single-phase coils connected together. The common point forms the electrical neutral point and may be grounded. Also referred to as three-phase four-wire. To obtain the line-to-neutral voltage, divide the line voltage by (1.732).


19 .3-16


EATON www.eaton.com

Frequently Asked Questions About Transformers

Can 60 Hz transformers be used at other frequencies?Transformers rated for 60 Hz can be applied to circuits with a higher frequency, as long as the nameplate voltages are not exceeded. The higher the frequency that you apply to a 60 Hz transformer, the less voltage regulation you will have. 60 Hz transformers may be used at lower frequencies, but only at reduced voltages corresponding to the reduction in frequency. For example, a 480–120 V 60 Hz transformer can carry rated kVA at 50 Hz but only when applied as a 400–100 V transformer (50/60 x 480 = 400).

Can single-phase transformers be used on a three-phase source?Yes. Any single-phase transformer can be used on a three-phase source by connecting the primary terminals of the single-phase transformer to any two wires of a three-phase system. It does not matter whether the three-phase source is three-phase three-wire or three-phase four-wire. The output of the transformer will be single-phase.

Can transformers be used to create three-phase power from a single-phase system?No. Single-phase transformers alone cannot be used to create the phase- shifts required for a three-phase system. Phase-shifting devices (reactors or capacitors) or phase converters in conjunction with transformers are required to change single-phase power to three-phase.

What considerations need to be taken into account when operating transformers at high altitudes?At altitudes greater than 3300 ft (1000 m), the density of the air is lesser than at lower elevations. This reduces the ability of the air surrounding a transformer to cool it, so the temperature rise of the transformer is increased. Therefore, when a transformer is being installed at altitudes greater than 3300 ft (1000 m) above sea level, it is necessary to derate the nameplate kVA by 0.3% for each 330 ft (100 m) in excess of 3300 feet.

What considerations need to be taken into account when operating transformers where the ambient temperature is high?Eaton’s dry-type transformers are designed in accordance with ANSI standards to operate in areas where the average maximum ambient temperature is 40 °C. For operation in ambient temperatures above 40 °C, there are two options:

1. Order a custom-designed transformer made for the specific application.

2. Derate the nameplate kVA of a standard transformer by 8% for each 10 °C of ambient above 40 °C.

What is the normal life expectancy of a transformer?When a transformer is operated under ANSI/IEEE basic loading conditions (ANSI C57.96), the normal life expectancy of a trans former is 20 years. The ANSI/IEEE basic loading conditions are:

A. The transformer is continuously loaded at rated kVA and rated voltages.

B. The average temperature of the ambient air during any 24-hour period is equal to 30 °C and at no time exceeds 40 °C.

C. The altitude where the transformer is installed does not exceed 3300 ft (1000 m).

What are Insulation Classes?Insulation classes were originally used to distinguish insulating materials operating at different temperatures. In the past, letters were used for the different designations. Recently, insulation system temperatures (°C) have replaced the letters’ designations.

Table 19.3-13. Insulation Classes PreviousDesignation

Insulation SystemRating (°C)

Class A 105

Class B 150

Class F 180

Class H 220

Class R 220

How do you know if the enclosure temperature is too hot?UL and CSA standards strictly regulate the highest tempera ture that an enclosure can reach. For ventilated transformers, the temperature of the enclosure should not increase by more than 50 °C in °C ambient at full rated current. For encapsulated transformers, the temperature of the enclosure should not increase by more than 65 °C in a 25 °C ambient at full rated current. This means that it is permissible for the temperature of the enclosure to reach 90 °C (194 °F). Although this temper ature is very warm to the touch, it is within the allowed stan dards. A thermometer should be used to measure enclosure temperatures, not your hand.

Can transformers be reverse-connected (reverse-fed)?Yes, with limitations. Eaton’s single- phase transformers rated 3 kVA and larger can be reverse-connected without any loss of kVA capacity or any adverse effects. Transformers rated 2 kVA and below, because there is a turns ratio compensation on the low voltage winding that adjusts voltage between no load and full load conditions, should not be reverse-fed.

Three-phase transformers with either delta-delta or delta-wye configurations can also be reverse-connected for step- up operation. When reverse-feeding a delta-wye connected transformer, there are two important considerations to take into account: (1) The neutral is not connected, only the three-phase wires of the wye system are connected; and (2) the ground strap between X0 and the enclosure must be removed. Due to high inrush currents that may be created in these applications, it is recommended that you do not reverse-feed transformers rated more than 75 kVA. The preferred solution is to purchase an Eaton step-up transformer designed specifically for your application.


19 .3-17


EATON www.eaton.com

Can transformers be connected in parallel?Yes, with certain restrictions. For single-phase transformers being connected in parallel, the voltages and impedances of the transformers must be equal (impedances must be within 7.5% of each other). For three-phase transformers, the same restrictions apply as for single-phase transformers, plus the phase shift of t he transformers must be the same. For example, a delta-wye–connected transformer (30° phase shift) must be connected in parallel with another delta-wye–connected transformer, not a delta-delta–connected transformer (0° phase shift).

Why is the impedance of a transformer important?The impedance of a transformer is important because it is used to determine the interrupting rating and trip rating of the circuit protection devices on the load side of the trans former. To calculate the maximum short-circuit current on the load side of a transformer, use the following formula:

Full load current for single-phase circuits is:

and for three-phase circuits the full load current is:

Example: For a standard three-phase, 75 kVA transformer, rated 480 V delta primary and 208Y/120 V secondary (catalog number V48M28T75J) and impedance equal to 5.1%, the full load current is:

The maximum short-circuit load current is:

The circuit breaker or fuse on the secondary side of this transformer would have to have a minimum interrupting capacity of 4083 A at 208 V. NEMA ST-20 (1992).

A similar transformer with lower impedance would require a primary circuit breaker or fuse with a higher interrupting capacity.

What clearances are required around transformers when they are installed?All dry-type transformers depend upon the circulation of air for cooling; therefore, it is important that the flow of air around a transformer not be impeded. Many Eaton transformers require a minimum clearance of 6 inches from panels with ventilation openings. However, small kVA DOE 2016 efficient ventilated transformers are UL approved to be installed with just 2 inches clearance, while large kVA transformers require 12 inches or more clearance. In compliance with NEC 450.9, Eaton’s ventilated transformers have a note on their nameplates identifying the minimum required clearance from the ventilation openings and walls or other obstructions. This clearance only addresses the ventilation needs of the transformer. There may be additional local codes and standards that affect installation clearances.

Transformers should not be mounted in such a manner that one unit will contribute to the additional heating of another unit, beyond allowable temperature limits, for example, where two units are mounted on a wall one above the other.

How Can I Reduce Transformer Sound Levels?All transformers emit some audible sound due mainly to the vibration generated in their core by alternating flux. NEMA ST-20 (2014) defines the maximum average sound levels for transformers.

Table 19.3-14. NEMA ST-20 (2014) Maximum Audible Sound Levels for 600 V Class Transformers (dBA)Equivalent Winding kVA Range

Average Sound Level, Decibels

Self-Cooled Ventilated Self-Cooled Sealed

A B C D

K Factor = 1K Factor = 4K Factor = 9

K Factor = 13K Factor = 20

Forced Air When Fans Running

3.00 and below3.01 to 9.009.01 to 15.00

404045

404045

676767

454550

15.01 to 30.0030.01 to 50.0050.01 to 75.00

454550

454853

676767

505055

75.01 to 112.50112.51 to 150.00150.01 to 225.00

505055

535358

676767

555557

225.01 to 300.00300.01 to 500.00500.01 to 700.00700.01 to 1000.00

55606264

58636567

67676767

57596163

Greater than 1000 Consult factory

Note: Consult factory for nonlinear requirements exceeding a K-factor rating of 20. When the fans are not running, columns A and B apply. Sound levels are measured using the A-weighted scale (dBA).

All Eaton transformers are designed to have audible sound levels lower than those required by NEMA ST-20 (2014). However, consideration should be given to the specific location of a transformer and its installation to minimize the potential for sound transmission to surrounding structures and sound reflection. The following installation methods should be considered:

1. If possible, mount the transformer away from corners of walls or ceilings. For installations that must be near a corner, use sound-absorbing materials on the walls and ceiling if necessary to eliminate reflection.

2. Provide a solid foundation for mounting the transformer and use vibration dampening mounts if not already provided in the transformer. Eaton’s ventilated trans formers contain a built-in vibration dampening system to minimize and isolate sound transmission. However, supplemental vibration dampening mounts installed between the floor and the transformer may provide additional sound dampening.

3. Make electrical connections to the transformer using flexible conduit.

4. Locate the transformer in an area where audible sound is not offensive to building inhabitants.

5. Install “low sound” transformers (up to 5 dB below NEMA ST-20 [2014] sound limits).


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Low-voltage dry-type mini–power centers · Eaton’s mini–power centers are available with a tin-plated aluminum bus loadcenter interior that accepts 1-inch plug-on branch circuit